(i) Field of the Invention
The present invention relates to a method for purifying a nitrogen trifluoride gas. More particularly, it relates to a method for removing nitrous oxide (N.sub.2 O), carbon dioxide (CO.sub.2) and dinitrogen difluoride (N.sub.2 F.sub.2) from a nitrogen trifluoride gas.
(ii) Description of the Prior Art
In recent year, a nitrogen trifluoride (NF.sub.3) gas attracts much attention as an etching agent for semiconductors and as a cleaning gas for CVD apparatus. The NF.sub.3 gas used in these applications is required to be of high-purity.
The NF.sub.3 gas can be prepared by various methods. For example, there are a process in which a molten salt of ammonium acid fluoride is electrolyzed, a process comprising the step of reacting gaseous fluorine with molten ammonium acid fluoride, a process in which fluorine in an elemental state is reacted with an ammonium complex of a solid metal fluoride, and a molten salt electrolysis process in a NH.sub.4 F.multidot.HF system or KF.multidot.NH.sub.4 F.multidot.HF system which are obtained from ammonium fluoride or acidic ammonium fluoride and hydrogen fluoride as well as potassium fluoride or acidic potassium fluoride. However, in most cases of the above-mentioned methods, the obtained gas contains impurities such as N.sub.2 O, CO.sub.2 and N.sub.2 F.sub.2 in relatively large amounts, and therefore purification is necessary, if the high-purity NF.sub.3 gas for the above-mentioned applications is desired.
A well known purification method for removing these impurities from the NF.sub.3 gas, which has been heretofore used, is a process for adsorbing/removing the impurities therefrom gas by the use of an adsorbent such as a synthetic zeolite, active carbon or active alumina.
In particular, the synthetic zeolite disclosed in U.S. Pat. No. 4,156,598 can adsorb the above-mentioned impurities efficiently, and thus the disclosed adsorbent is considered to be satisfactory to some extent. However, according to the discoveries of the present inventors, the synthetic zeolite, for example, Molecular Sieve 5A has the problem that its ability to adsorb N.sub.2 O is great, but its adsorbability to CO.sub.2 is poor. In the case of Molecular Sieve 13X, inversely its ability to adsorb CO.sub.2 is great, but its adsorbability to N.sub.2 O is poor. In short, the ability of the synthetic zeolite to adsorb the impurities depends upon a kind of synthetic zeolite itself.
Accordingly, the inventors of the present case have found that when an attempt is made to remove the abovementioned impurities by adsorbing, two or more kind of synthetic zeolites must be used simultaneously, which is troublesome and uneconomical.
In addition, it has been also found that the synthetic zeolite adsorbs not only the impurities but also NF.sub.3 which is the product, with the result that a great deal of the NF3 gas is lost very inconveniently.
On the other hand, when active carbon or active alumina is used as the adsorbent, the adsorption of the impurities per unit volume of such a kind of adsorbent is weaker than when the synthetic zeolite is used, though the adsorption of NF.sub.3 is also relatively weak. Therefore, the adsorbability of the active carbon or active alumina goes away in an extremely short period of time. In consequence, the renewal or recovery of the adsorbent must often be made, and at the time of this renewal or recovery, the NF.sub.3 gas is lost. After all, not a little volume of the NF.sub.3 gas is lost on the whole. This problem has also been found by the present inventors.
Moreover, in purifying the NF.sub.3 gas, it requires much labor and much time to often replace or regenerate the adsorbent.
It means that purifying capacity is substantially lowered.
In view of such situations, the present inventors have intensively conducted researches on techniques for removing N.sub.2 O, CO.sub.2 and N.sub.2 F.sub.2 from the NF.sub.3 gas, and as a result, it has been found that when the NF.sub.3 gas having a specific temperature is passed through the natural zeolite packed bed which has been beforehand thermally treated at a specific temperature, a less amount of NF.sub.3 is merely adsorbed by the natural zeolite, and the above-mentioned impurities can be removed efficiently and economically from the NF.sub.2 gas, since the natural zeolite has a high adsorbability to N.sub.2 O, CO.sub.2 and N.sub.2 F.sub.2. On the bases of this knowledge, the present invention has been achieved.